1. Field of the Invention
The present invention relates to a flexible printed circuit board, and more particularly to a flexible printed circuit board for electrical connection by wire bonding and an ink jet recording head using the same.
2. Related Background Art
As the size of an electronic equipment is reduced, a flexible printed circuit board (FPC) is widely used in view of the freedom of packaging form and the space saving. In connecting the FPC and other wiring material or an electronic part mounted on the FPC, a wire bonding (WB) is frequently used when a high density packaging is required or a packaging space is limited.
An ink jet recording head is explained as an example of using the WB for the connection of the FPC.
FIG. 8 shows a sectional view of a packaging unit of an ink jet recording head previously proposed by the inventors of the present invention, as an example of the use of the WB for the connection of the FPC.
FIGS. 4A and 4B show perspective views of the ink jet recording head of FIG. 8.
An overall construction of the ink jet recording head is explained with reference to FIGS. 4A, 4B and FIG. 8.
On an upper surface of a support plate 5 made of aluminum, a heater board 4 having an Si substrate on which a heater for applying a thermal energy to ink is formed and one end of an FPC 1 for transmitting an electrical signal sent from a main unit of a recording apparatus to the heater board 4 are electrically connected by a wire bonding 12. The FPC 1 is bent toward a rear surface (lower surface) of the support plate 5 and the other end of the FPC 1 is connected to the support plate 5. Contact pads 111 which are electrically connected to a contact pad of the main unit of the recording apparatus are formed on the FPC 1. The ink jet recording head is supplied with the electrical signal from the main unit of the recording apparatus through the contact pads 111 on the FPC 1. The support plate 5 is covered with a packaging cover 9 and ink is supplied from the main unit of the recording apparatus to a grooved top plate 8 having a common liquid chamber and discharge ports 10 forming nozzles through an ink supply member 11 provided in the packaging cover 9, and it is heated by the heater on the heater board 4 and discharged from the discharge ports 10. Sealing agent 10 such as silicone rubber is applied to the connecting area of the wires 12 to protect the bonding part from an external force.
A layer structure of the WB connecting part of the FPC 1 is now explained with reference to FIG. 7.
The layer structure in the connecting part of the FPC 1 usually comprises, starting from a bottom layer, reinforcing plate 101 made of glass epoxy and provided only on the support member, a bonding layer 108 made of a acryl resin, a base film 103 made of polymide, a bonding layer 104 made of epoxy resin, a conductive layer 105 (10-35 .mu.m) made of a copper foil, a nickel plating layer 109 (2-3 .mu.m), a gold plating layer 107 (1 .mu.m) serving as a contact member, in this order. Since the nickel plating layer is provided to enhance the contact of the copper of the conductive layer and the gold of the contact member, a thickness of 2-3 .mu.m is sufficient.
The FPC 1 is bonded to the substrate 5 by the bonding material 2a applied to the reinforcement plate 101. Other printed circuit board 7 is also bonded to the substrate 5 by the bonding material 2b.
The gold plating layer 107 of the FPC 1 and the WB pad 701 of the other printed circuit board 7 are electrically connected by a gold wire 12. The gold wire, the gold plating layer 107 and the WB pad 701 are ball-bonded.
In such an FPC, where a number of wires are required as it is in an ink jet recording head, aluminum wires may be used for the WB because the gold wires are expensive.
However, when the aluminum wires are used for the WB in the prior art FPC, defects in the connection are frequently found and a yield is very low.